ammonium nitrate in 1 :1 acetone water. After washing, the precipitate can be ignited at 1100" C or redissolved for further treatment. RESULTS AND DISCUSSION
The results of the study are presented as decontamination factors calculated from peak height ratios in Table I. The spectra from which the peak heights were estimated are presented in Figure 1. It is concluded that the method provides
excellent separation of zirconium and niobium from each other and from many other elements which would be found in fission product mixtures. Although it has not been demonstrated, it is thought that ignited niobium phosphate, when precipitated from an excess of phosphate, would make a good weighing form for the determination of recovery (4). RECEIVED for review September 11, 1967. Accepted December 12, 1967.
Cartridge-Type Vacuum Lock for a Thermal-Ionization Mass Spectrometer Rapid Determination of Relative Isotopic Ratios of Potassium Olin H . Howard, Aubrey Langdon, and Clint Sulfridge Oak Ridge Gaseous Diffusion Plant, Union Carbide Corp., Nuclear Division, Oak Ridge, Tenn. AN EXPERIMENTAL PROCESS for the separation of potassium isotopes ( I ) requires frequent determinations of the relative ratios of 4IK to 39K among the various stages of the process. Relative isotopic ratios, defined as follows, are determined by thermal-ionization mass spectrometry:
where R = relative isotopic ratio, ra = a1K/39K (Sample a), and rb = 41K/39K(Sample 6). Many of the works which have been published on the determination of potassium isotope abundances (2-6) have concerned the accurate measurement of the isotopic composition of natural potassium. The purpose of the work reported here is to determine rapidly the relative isotopic ratios of potassium altered in an experimental system. INSTRUMENTATION
A double-filament thermal-ionization source and a cartridge-type vacuum lock (Figure 1) were developed for a 60" magnetic, 6-inch radius mass spectrometer. Potassium iodide is vaporized from a sample filament. Potassium ions are generated when the vapor contacts a hotter, ionizing filament. The ions are accelerated by 3000 volts through a 0.020-inch beam-defining slit and through the magnetic analyzer. The 41K+ and 3QK+ion currents are detected simultaneously on separate collectors, amplified, and fed to a ratio recorder (7) whose output is proportional to the 41K/39K ratio. (The small amount of 40K is detected and measured with the 39K.) The input resistors to the high-current (mass (1) R. M. McGill, R . W. Browell, J. W. Grisard, S. Blumkin, E. VonHalle, and D. B. Janney, U . S. At. Energy Comm. Rept. K-1650 (1965). (2) A. Keith Brewer,J. Am. Chem. Soc., 59,869 (1937). (3) B. R. F. Kendall, Nurure, 186,225 (1960). (4) A. 0. Nier, Phys. Rec., 77, 789 (1950). (5) Carl Reutersward, Arkic. Fysik., 11, 1 (1956). (6) J. R . White and A. E. Cameron, Phys. Rer;., 74, 991 (1948). (7) W. G . Hart and A. Langdon, U . S. At. Energy Comm. Rept. K-1292 (1959).
39) and low-current (mass 41) direct-current amplifiers are 3 x 10'0 ohms. The vacuum in the source chamber is obtained with an 80liter-per-second ion pump and a liquid nitrogen cold finger. The source pressure during an analysis is usually between 5 x 10-6 and 5 X lo-' torr. The analyzer is pumped with an 8-liter-per-second ion pump to pressures less than 10-7 torr during an analysis. Ion Source. The double-filament ion source, a variation of the triple-filament source (8),includes a rhenium ionizing filament (0.001 inch thick X 0.030 inch wide X 0.5 inch long) and a platinum sample filament (0.004 inch thick X 0.030 inch wide x 0.5 inch long). The ionizing filament is mounted on the first plate of the ion lens assembly. The sample filament is mounted on a cylindrical cartridge (OS-inch diameter x 3.5 inches long) which is inserted into the source chamber through a vacuum lock. Two filaments are used instead of one for several reasons: the 10-volt input signal required by the ratio recorder can be more efficiently maintained with a multiple-filament than with a single-filament source; the ion current from a multiplefilament source is more constant than that from a singlefilament source; and the analytical precision is probably better when the ionizing filament remains undisturbed in the source during sample changes, thereby retaining constant ionization-region geometry from sample to sample. The magnitude of the ion current is strongly dependent upon the positions of the filaments. The ionizing filament must be near and in line with the source entrance slit. The sample filament must be near the ionizing filament, and in front of it relative to the direction of ion flow, as shown in Figure 2. The ionizing filament is approximately 0.125 inch from the source entrance slit. The sample filament is positioned during each analysis, by appropriate rotation of the cartridge, for maximum ion current. To minimize warping or sagging of the ionizing filament with use, it is welded to its terminals under tension. The terminals are first spread wider than the length of the filament and then sprung together sufficiently as the filament is welded to them. (8) Mark G. Inghram and William A. Chupka, Rec;. Sci. Insrr., 24, 518 (1953). VOL. 40, NO. 3, MARCH 1968
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MBER CONTACTOR CONTROL
FILAMENT CONTACT0 T Z ELECTRICAL INSULATOR
SAMPLE FllAMEN SAMPLE CARTRIDG PUMPING STAGES
SOURCE PUMP LINE
INSERTION CHAMBER
NlZlNG FILAMENT
Figure 1. Vacuum lock The filaments are heated with direct current ( 2 A ionizing and 2 to 2.5 A sample) from a current-regulated power supply. Filament temperatures are not measured, but the sample filament is probably less than 500" C and the ionizing filament less than 1000" C for potassium analysis. A given filament, either ionizing or sample, lasts indefinitely. A new rhenium ionizing filament contains some potassium as an impurity and therefore emits potassium ions, but these are eliminated by heating the filament for a few minutes above its operating temperature. A clean platinum sample filament does not emit significant potassium at its operating temperature. A liquid nitrogen cold finger in the source region accelerates reduction of the source pressure by trapping condensables introduced with the sample. A copper strap, interrupted by a 1-inch length of OS-inch-diameter quartz rod for electrical insulation, connects the cold finger and the first plate of the ion lens assembly, cooling the latter to -60" C (estimated) during operation of the mass spectrometer. Vacuum Lock. The vacuum lock (Figure l), a variation of the sliding-bar type (9), includes a stainless-steel cylinder 0.5-inch nominal i.d. and 8 inches long leading into the source and a similar one leading out. At the entrance and exit ends of the lock are 4-inch-long insertion and extraction chambers, respectively. Each end of the lock is pumped in four stages. The outer three stages, including the insertion and extraction chambers, are rough-pumped, and the innermost stage is evacuated with an ion pump (15 liters/second, water-cooled). The axis of the lock is parallel to the slits and plates of the ion lens assembly. A sample cartridge in the source chamber is approximately inch from the source entrance slit and from the first plate. The lock cylinders are attached to the source housing with Teflon-gasketed flanges. The cylinders extend into the 3.5inch-diameter source chamber approximately 0.75 inch, sufficient for a sample cartridge to span the gap between the two, yet far enough from the source for electrical insulation. During routine operation, the entrance end of the lock contains two samples awaiting analysis, the source chamber contains one being analyzed, and the exit end contains two which have been analyzed. The sample cartridges are moved through the lock and (9) C. M. Stevens, Rec. Sci. Instr., 24, 148 (1953).
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u-2
Z i Y R C E ENTRANCE
SAMPLE FllAMENT
I
Figure 2.
Filament positions
rotated as required with a 0.5-inch-diameter x 4-inch-long, tongued push rod. Threaded, neoprene-gasketed nuts provide the vacuum seals for the push rod, the entrance of the insertion chamber, and the extraction chamber. When an analysis has been completed, the insertion chamber is valved off from its pump, a freshly loaded cartridge is inserted, and the chamber is re-evacuated. The cartridge train is pushed until the next sample is in the source chamber and rotated until the sample filament is in the approximately correct position as viewed through a sight glass. The extraction chamber is valved off from its pump, a used cartridge is removed, and the chamber is re-evacuated, Outgassing of the sample in the source chamber, requiring approximately 10 minutes, is begun. Although the vacuum lock is small and has been used only on a mass spectrometer at an ion-accelerating potential of 3000 volts, the high-voltage portion of the sample cartridge is a sufficient distance from electrical ground to permit considerably higher ion-accelerating potentials. Sample Cartridge. The sample cartridge includes a 1inch-Iong stainless-steel filament-assembly mount; two 1inch-long porcelain insulators, one screwed to each end of the mount, and two Teflon O-rings, one held in place at one end of the cartridge by a grooved stainless-steel screw and one at the other end by a tongued screw. The Teflon O-rings provide bearing surfaces for sliding the cartridge through the vacuum lock and also provide seals between the pumping stages of the lock. The tongue-and-groove design of the cartridge ends enables rotation of the cartridge train in the vacuum lock so that the sample filament can be properly positioned with respect to the ionizing filament.
The sample filament is spring-loaded so that it is normally recessed within a depression in its mount. When the cartridge is in the analyzing position in the source chamber, the filament is extended within I/a2 inch of the ionizing filament and connected to the filament-current supply and high voltage by a contactor which is controlled through a bellows seal. The filament assemblies of used cartridges are washed in hot water, and vacuum-dried at 120' C for reuse. DETERMINATION OF RELATIVE ISOTOPIC RATIOS OF POTASSIUM
Procedure. A potassium metal sample is hydrolyzed, neutralized with hydriodic acid, and diluted to give a solution of potassium iodide containing 1 mg of potassium per ml. One drop, containing approximately 20 pg of potassium, is placed on the filament of a sample cartridge. The filament is dried by passing 5 A ac through it and inserted into the mass spectrometer. While maintaining a 10-volt 39K signal a ratio-recorder reading is obtained. The comparative sample is then similarly analyzed, followed by a repeat analysis of the first sample. (An analysis of one of the samples of a pair is bracketed within two analyses of the other sample to compensate for any systemic drift.) The 41K/39Krelative ratio for the two samples is calculated by dividing the higher average reading by the lower. The mass spectrometer adjustments are disturbed as little as possible between comparative samples. The ionizingfilament current and the accelerating voltage are left on, and the ion-lens potentials are not disturbed. Sample Form and Size. The sample must vaporize from the sample filament at the required rate at a lower temperature than that at which it ionizes. Otherwise the ions originate at the sample filament instead of the ionizing filament. For example, if the sample is potassium phosphate (mp 1340' C) essentially all of the ions originate at the sample filament. On the other hand, if the sample is potassium iodide (mp 723" C), all of the ions originate at the ionizing filament. A sample size of 20 pg of potassium is arbitrarily chosen; however, experimental measurements have been obtained on as little as 0.2 pg. Effect of Varying Ion-Lens Potentials. The ion-lens potentials are the same for all analyses, and no electrostatic beam centering is used. Operating this way, a relative standard deviation of 0.2% is obtained us. 1 % when the source is refocused for maximum ion current during each analysis. The sensitivity is ample despite a 30% loss incurred by not refocusing. Cross Contamination. If the ion lens assembly is not cooled, it becomes hot during continuous operation and vaporizes potassium iodide deposited on it from previous samples, resulting in cross contamination. This is evidenced by a decrease in the indicated relative ratio of a pair of samples analyzed repeatedly without interruption and by a low indicated relative ratio compared with that obtained by a different mass spectrometer. The cross contamination is minimized by keeping the ionizing-filament temperature as low as possible and is reduced to an undetectable level by cooling the source. Three-hundred samples at 41K/39K
Table I. Precision of Analysis 41K/8gKrelative ratio Determination No. Sample pair #1 Sample pair #2 1 1.539 1,0047 2 1.534 1.W91 3 1.536 1.0083 1.540 4 1.0052 5 1.539 1.0059 6 1.537 1.0057 7 1.536 1,0038 8 1.535 1.0045 9 1.533 1.OO67 Mean 1,5366 1,0060 Std dev 0.0024 0.0018 Re1 std dev, 0.16 0.18 relative ratios as high as 1.5 have been analyzed without cleaning the source, and with no evidence of cross contamination. RESULTS AND DISCUSSION
Precision and Accuracy. Standard deviations of 0.0018 and 0.0024 are obtained at 41K/39Krelative ratios of 1.0060 and 1.5366, respectively; hence approximately 0.2% relative standard deviation (Table I). Only Chorlton (IO), who loaded both samples on the same triple-filament source, has reported equivalent precision. Agreement with a standard 12-inch radius thermal-ionization mass spectrometer is accepted as indicating no significant bias in the method. relative ratio, requiring three Analysis Rate. One 41K/39K sample loadings, is determined per hour. Potential for Accurate Measurement of Absolute Isotopic Ratios. In our work, the information needed is the ratio of isotopic ratios of pairs of samples. Absolute ratios can be determined by using isotopic standards for comparison. It is seen from the precision and the analysis rate that the 39K/41Kratio of a sample of natural potassium, which is approximately 14, can be determined within 0.03 (standard deviation) within 1 hour if a sufficiently accurate standard is used. ACKNOWLEDGMENT
L. A. Smith proposed the cartridge-type vacuum lock, and E. L. Benedict assisted in its design. RECEIVED for review June 14, 1967. Accepted December 15, 1967. Work performed at the Oak Ridge Gaseous Diffusion Plant operated by Union Carbide Corporation for the U. S. Atomic Energy Commission. Presented at Fifteenth Annual Conference on Mass Spectrometry and Allied Topics, Denver, Colo., May 1967.
(10) S. H.Chorlton, J. Sci. Insrr., 43, 200 (1966).
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